A conventional linear actuator shown in FIG. 26 is hereinafter described. This linear actuator comprises the following elements:                inner yoke 10, namely, a laminated core formed of numbers of E-shaped magnetic plates 8 punched out from thin steel plate by a die and the E-shaped sheet being arranged in a cylindrical shape radiating around a center axis;        coil 2 wound on slot 1 of inner yoke 10;        outer yoke 3 formed of numbers of I-shaped magnetic sheets punched out from rectangular thin steel plate punched by a die, and the I-shaped sheets being arranged in a cylindrical shape radiating around the center axis;        permanent magnets 5 and 6 disposed in a gap between inner yoke 10 and outer yoke 3; and        vibrator 7 for supporting permanent magnets 5 and 6.Permanent magnets 5 and 6 are magnetized in radial direction, and the magnetic poles of those magnets are arranged such that the inner yoke side of magnet 5 is N pole and the inner yoke side of magnet 6 is S pole. The magnetic poles so arranged, i.e., opposite poles of each magnet, are fixed to vibrator 7.        
In the linear actuator structured above, electric current flowing through coil 2 generates magnetic flux that forms magnetic paths indicated with arrow marks. Changing a direction of the electric current changes a direction of the magnetic flux flowing from coil 2, and magnets 5, 6 repeat attraction and repulsion responsive to the change of the magnetic flux. As a result, magnets 5 and 6 reciprocate along the axial direction.
The conventional linear actuator, however, has the following problems:
(1) Since the conventional laminated cores described above are laminated cylindrically, each magnetic sheet should be thicker at the outer side of the core and thinner at the inner side of the core. A magnetic sheet available in the market; however, is constant in thickness across the sheet, thus each one of the magnetic sheets seems to be cut for changing the thickness to be assembled into the conventional laminated cores. This method is not fit to volume production, or cannot keep the thickness uniform throughout all the magnetic sheets. Thus it is difficult to form a cylindrical shape with the conventional magnetic sheets.
(2) Although it is not shown in the drawings, even if a laminated core is produced with magnetic sheets having a uniform thickness across the sheet, the adjacent sheets are contact with each other at inner side. However, they have gaps between at outer side, and bonding such as applying varnish between the sheets and supporting members is required. As a result, the cost increases substantially. Since this laminated core is a radial laminated body, the magnetic sheets radiate from a center, so that the gap becomes wider toward the outer rim. Thus the total iron amount of the inner yoke, i.e., space factor, lowers, and it is difficult for the magnetic flux to travel from the permanent magnets to the thin steel plate.
(3) In the case of a conventional C-shaped or E-shaped core, the magnetic flux generated from the coil travels along the vibrating direction on the inner wall side and radial direction on the opposite side to the permanent magnet. More efficient usage of the magnetic flux needs to employ electrical steel sheet of which magnetic property tends to feed magnetic flux in one direction. This oriented electrical steel sheet tends to feed the magnetic flux in the rolling direction, in fact the magnetic property lowers along right angles with respect to the rolling direction. Therefore, in the case of using the steel plate punched out by a die into C-shaped or E-shaped cores, either one of a vibrating direction or a radial direction is to use a magnetic property having a lower permeability along right angles with respect to the rolling direction. The magnetic flux generated from the coil thus cannot be efficiently used.